Rotary rheometer magnetic bearing

ABSTRACT

A rotary rheometer having a magnetic thrust bearing. The thrust disk of the magnetic thrust bearing being situated between a pair of magnetic actuator assemblies and extending beyond the circumference of the actuator assemblies so as to encompass the magnetic flux lines generated by the actuator assemblies, thus minimizing likelihood of an undesirable preferential position or side load.

This Application is a Continuation of and claims priority to U.S.Application Ser. No. 11/075,414 entitled Rotary Rheometer MagneticBearing, filed on Mar. 9, 2005 now U.S. Pat. No. 7,017,393, which claimsthe benefit of U.S. Provisional Application No. 60/551,802, filed Mar.11, 2004.

BACKGROUND

1. Field of the Invention

The present invention relates generally to rheometers, which are used tocharacterize materials by measuring the materials' viscosity,elasticity, shear thinning, yield stress, compliance and/or othermaterial properties.

2. Background of the Invention

Rotary rheometers, viscometers or viscosimeters are used to measurefluid or other properties of materials such as their viscosity byrotating, deflecting or oscillating a measuring object in a material,and measuring, for example, the torque required to rotate or deflect oroscillate the object within the material. As used herein, the term“rheometer” shall mean rheometers, viscometers, viscosimeters andsimilar instruments that are used to measure the properties of fluid orsimilar (see list below) materials.

The term “measuring object” shall mean an object having any one ofseveral geometries, including, for example, cones, discs, vanes,parallel plates, concentric cylinders and double concentric cylinders.The materials may be liquids, oils, dispersions, suspensions, emulsions,adhesives, biological fluids such as blood, polymers, gels, pastes,slurries, melts, resins, powders or mixtures thereof. Such materialsshall all be referred to generically as “fluids” herein. More specificexamples of materials include asphalt, chocolate, drilling mud,lubricants, oils, greases, photoresists, liquid cements, elastomers,thermoplastics, thermosets and coatings.

As is known to one of ordinary skill in the art, many differentgeometries may be used for the measuring object in addition to thecylinders, cones, vanes and plates listed above. The measuring objectsmay be made of, for example, stainless steel, anodized aluminum ortitanium. U.S. Pat. No. 5,777,212 to Sekiguchi et al., U.S. Pat. No.4,878,377 to Abel and U.S. Pat. No. 4,630,468 to Sweet describe variousconfigurations, constructions and applications of rheometers.

FIG. 1A is a schematic perspective view of a prior art rotary rheometer100, showing lead screw 101, draw rod 102, optical encoder 103, airbearing 104, drive shaft 105, drag cup motor 106, measuring object 107(shown in FIG. 1A as a parallel plate), heating/cooling assembly (e.g.,a Peltier plate) 108, temperature sensor 110 (e.g., a Pt temperaturesensor), surface 111, normal force transducer 112, and auto gap setmotor and encoder 113. FIG. 1B is a schematic drawing of a concentriccylinder configuration in position on the rheometer of FIG. 1A, showingthe control jacket 120 of the concentric cylinder configuration on topof normal force transducer 112 of rheometer 100. FIG. 1B shows acylindrical measuring object 121 (used in this configuration instead ofthe parallel plate measuring object 107 shown in FIG. 1A.

Typical rheometers include essentially two types of bearings formaintaining the position of the shaft, radial bearings and thrustbearings. Modern rheometers utilize air (or other mechanical) bearingsfor both the thrust and radial bearings because they are non-contact andlow friction. The viscosity of high-pressure air in the bearing is oneof the limiting factors to the lowest torques that may be applied by themotor, while still resulting in accurate data. One such alternativewould be to use a bearing that levitates magnetically.

In rheometers, magnetic bearings have not been fully commercialized. Onemagenetic bearing that has been utilized in rheometer applications wasdescribed by Don Plazek in 1968 (“Magnetic Bearing Torsional CreepApparatus,” Journal of Polymer Science, A2 6:621–638). This magneticbearing utilized a combination thrust and radial bearing of a cone andring shape. Such a magnetic bearing has alignment and preferentialposition issues and its design is not considered robust enough fortypical laboratory use. In addition, this rheometer did not provide thefull spectrum of capabilities of typical modern rheometers in that itcould only be used to measure creep and was not suitable for otherapplications such as, for example, steady shear, dynamic and stressrelaxation.

SUMMARY OF THE INVENTION

Magnetic bearings are well known in other areas, but there areparticular requirements for a rotary rheometer. The bearing should nothave a preferential position or apply a side load that could result inundesirably interactions with radial bearings. If the pole pieces of theelectromagnets and thrust disk are exactly the same size and perfectlyaligned, then the magnetic flux lines will cut vertically through thedisk in a symmetrical manner. If, however, there is any misalignment ordifference in size, a preferential position or side load can result. Inorder to overcome these issues, the present invention utilizes a thrustdisk sized so as to be larger than the exterior and interiorcircumferences of the pole-pieces of the magnetic bearing. In thisconfiguration, there is always thrust disk material for the magneticflux lines to pass through, thus minimizing the likelihood of apreferential position or a side load.

The magnetic thrust bearing of embodiments of the present invention isdisk shaped and may be used in conjunction with two separate radialbearings, for example radial air bearings, that impart a radialstiffness and robustness. It is believed that this configuration mayopen the use of magnetic thrust bearings to many more applicationsbecause of the robustness of the overall design and lower cost ofmanufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a perspective view of a prior artrotary rheometer.

FIG. 1B is a schematic diagram of a concentric cylinder configuration inposition on the rheometer of FIG. 1A.

FIG. 2 is a schematic diagram of a perspective view of a rotaryrheometer according to an exemplary embodiment of the present invention.

FIG. 3 is a schematic diagram of the upper portion of the rotaryrheometer of FIG. 2.

FIG. 4 is a partial cross section of the magnetic thrust bearingdepicted in FIGS. 2 and 3.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 is a perspective cut-away schematic diagram that shows anexemplary embodiment of a rheometer shaft housing 200 comprising amagnetic thrust bearing assembly 210 of the present invention. Housing200 houses rotary shaft 202, which is rotated by motor assembly 220.Motor assembly 220 comprises a pair of radial bearings 222 and 224.Radial bearings 222 and 224 comprise conventional air bearings, butcould comprise other mechanical bearings known in the art.

In addition to radial bearings 222 and 224, housing 200 also comprisesthe thrust bearing assembly 210. Unlike conventional air or mechanicalbearings, thrust bearing assembly 210 is a magnetic thrust bearing. Asseen in FIG. 3, which is a zoomed in drawing of the thrust bearingassembly 210 depicted in FIG. 2, thrust bearing assembly 210 comprises apair of magnetic actuator assemblies or magnets 212 a and 212 b. Betweenmagnets 212 a and 212 b is situated thrust disk 214. Thrust disk 214 maybe made of, for example, magnetic iron. In the embodiment shown, magnets212 a and 212 b are attractive magnets and thus maintain the position ofthrust disk 214 between them. This in turn maintains the verticalposition of the shaft within motor housing 200.

As seen in FIG. 4, a partial cross-section of thrust bearing assembly210, thrust disk 214 is larger than the two magnets 212 a and 212 b.This increased size allows substantially all of the magnetic flux linesto cut vertically through the disk. It may be possible to manufactureelectromagnets so that they are exactly the same size as the thrustdisks, but such a thrust disk would be nearly impossible to align andassemble, thus greatly increasing manufacturing and operational costs.Any such misalignment can result in an undesirable preferential positionor sideload, thus requiring constant adjustment and/or recalibration.

Accordingly, thrust disk 214 is of a sufficient size so as to compensatefor and always provide thrust disk material for the magnetic flux linesto pass through. This, in turn, minimizes the likelihood of apreferential position or sideload. One of skill in the art willunderstand that the proportions of the disk versus the magnets shown isonly exemplary and is not to scale and that the thrust disk need only beof a sufficient size in comparison to the magnets to encompass the fluxlines. Optimization of the magnet size versus disk size is alsocontemplated by the present invention so that torque performance can bemaximized while still minimizing moment of inertia and maintainingproper axial, lateral, and torsional stiffness.

Other advantages that can be achieved by use of a magnetic thrustbearing over an air thrust bearing are that the bearing gap can beincreased, for example, to approximately 0.5 mm on each side rather thanthe microns associated with an air bearing. The magnetic bearing alsoreduces friction and residual torque resulting in ultra-low usabletorques. The magnetic thrust bearing is also more robust and lesssusceptible to contamination.

As an example of the improved characteristics, the following table showscomparative specifications of the AR-G2 rotary rheometer having themagnetic thrust bearing of the present invention versus the AR 2000rotary rehometer (both manufactured by TA Instruments, Inc.) havingconventional air bearings only. In the following chart, CR stands forcontrolled rate mode and CS stands for controlled stress mode:

Specification AR-G2 AR 2000 Minimum torque: oscillation CR 0.003 μNm0.03 μNm Minimum torque: oscillation CS 0.003 μNm  0.1 μNm Minimumtorque: steady shear CR  0.01 μNm 0.05 μNm Minimum torque: steady shearCS  0.01 μNm  0.1 μNm

The foregoing disclosure of the preferred embodiments of the presentinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Many variations andmodifications of the embodiments described herein will be apparent toone of ordinary skill in the art in light of the above disclosure. Thescope of the invention is to be defined only by the claims appendedhereto, and by their equivalents.

1. A rheometer comprising: a rotary shaft; a motor assembly for rotatingthe rotary shaft, wherein the motor assembly comprises at least oneradial bearing; and a magnetic thrust bearing having a thrust diskcoaxial with the rotary shaft and positioned between two magnets,wherein the thrust disk is configured to allow substantially all ofmagnetic flux lines generated by the magnets to cut through the thrustdisk.
 2. The rheometer of claim 1, wherein an outer diameter of thethrust disk is of sufficient size to encompass the magnetic flux lines.3. The rheometer of claim 1, wherein an outer diameter of the thrustdisk is larger than an outer diameter of the magnets.
 4. The rheometerof claim 1, wherein the magnets exert attractive force upon the thrustdisk.
 5. The rheometer of claim 1, wherein the at least one radialbearing comprises an air bearing.
 6. The rheometer of claim 1, wherein abearing gap between the thrust disk and at least one of the magnets isapproximately 0.5 mm.
 7. The rheometer of claim 1, wherein a bearing gapbetween the thrust disk and each of the magnets is approximately 0.5 mm.8. The rheometer of claim 1, wherein the thrust disk comprises magneticiron.
 9. A magnetic thrust bearing for a rotary shaft, comprising: apair of magnets; and a thrust disk coaxial with the rotary shaft andpositioned between the pair of magnets, wherein the thrust disk isconfigured to allow substantially all of magnetic flux lines generatedby the pair of magnets to cut through the thrust disk.
 10. The magneticthrust bearing of claim 9, wherein an outer diameter of the thrust diskis of sufficient size to encompass the magnetic flux lines.
 11. Themagnetic thrust bearing of claim 10, wherein an outer diameter of thethrust disk is of sufficient size to always encompass the magnetic fluxlines.
 12. The magnetic thrust bearing of claim 9, wherein an outerdiameter of the thrust disk is larger than an outer diameter of the pairof magnets.
 13. The magnetic thrust bearing of claim 9, wherein the pairof magnets exert attractive force upon the thrust disk.
 14. The magneticthrust bearing of claim 9, wherein a bearing gap between the thrust diskand at least one of the pair of magnets is approximately 0.5 mm.
 15. Themagnetic thrust bearing of claim 9, wherein a bearing gap between thethrust disk and each of the pair of magnets is approximately 0.5 mm. 16.A rotary motor, comprising: a rotary shaft; at least one radial bearing;and a magnetic thrust bearing comprising a thrust disk coaxial with therotary shaft and positioned between two magnets, wherein the thrust diskis configured to allow substantially all of magnetic flux linesgenerated by the magnets to cut through the thrust disk.
 17. The rotarymotor of claim 16, wherein an outer diameter of the thrust disk islarger than an outer diameter of the two magnets.
 18. The rotary motorof claim 16, wherein the outer diameter of the thrust disk is ofsufficient size to always encompass the magnetic flux lines generated bythe magnets.
 19. The rotary motor of claim 16, wherein the magnets exertattractive force upon the magnetic disk.
 20. The rotary motor of claim16, wherein a bearing gap between the thrust disk and each of themagnets is approximately 0.5 mm.